Illuminating dark photons with high-energy colliders

Abstract

High-energy colliders offer a unique sensitivity to dark photons, the mediators of a broken dark U(1) gauge theory that kinetically mixes with the Standard Model (SM) hypercharge. Dark photons can be detected in the exotic decay of the 125 GeV Higgs boson, h→ZZ _D →4ℓ, and in Drell-Yan events, pp→Z _D → ℓℓ. If the dark U(1) is broken by a hidden-sector Higgs mechanism, then mixing between the dark and SM Higgs bosons also allows the exotic decay h → Z _D Z _D → 4ℓ. We show that the 14 TeV LHC and a 100 TeV proton-proton collider provide powerful probes of both exotic Higgs decay channels. In the case of kinetic mixing alone, direct Drell-Yan production offers the best sensitivity to Z _D , and can probe ϵ ≳ 9 × 10⁻⁴ (4 × 10⁻⁴) at the HL-LHC (100 TeV pp collider). The exotic Higgs decay h → ZZ _D offers slightly weaker sensitivity, but both measurements are necessary to distinguish the kinetically mixed dark photon from other scenarios. If Higgs mixing is also present, then the decay h → Z _D Z _D can allow sensitivity to the Z _D for ϵ ≳ 10⁻⁹ − 10⁻⁶ (10⁻¹⁰ − 10⁻⁷) for the mass range $2{m}_{\mu }<{m_Z}_{{}_D}<{m}_h/2$ by searching for displaced dark photon decays. We also compare the Z _D sensitivity at pp colliders to the indirect, but model-independent, sensitivity of global fits to electroweak precision observables. We perform a global electroweak fit of the dark photon model, substantially updating previous work in the literature. Electroweak precision measurements at LEP, Tevatron, and the LHC exclude ϵ as low as 3 × 10⁻². Sensitivity can be improved by up to a factor of ∼ 2 with HL-LHC data, and an additional factor of ∼ 4 with ILC/GigaZ data.